E. coli O157:H7 is a virulent food-borne pathogen that causes acute hemorrhagic colitis (bloody inflammation of the colon), and sometimes a severe hemolytic uremic syndrome (HUS), in children. These conditions can be fatal, or cause permanent kidney damage and shortened life expectancy. Treatment options for this infection are limited; prevention of human infection is therefore of the greatest importance.
Epidemics of E. coli O157:H7 generally have been traced to contaminated water or meats, especially undercooked hamburger. Following the 1993 massive outbreak in Washington State, the strain responsible for the epidemic disappeared soon after the recall of the incriminated vehicle. Hence, ingestion of contaminated beef, and not person to person spread, appears to be the chief source of human infection.
Because of the rapid onset of life-threatening complications, expedient diagnosis is of utmost importance to those infected with this strain of E. coli, especially when the patient is a child. However, because the early symptoms of this disease can mimic other disorders, patients infected with this organism during the initial stages of new epidemics may not be tested for E. coli O157:H7 until other causes have been ruled out. Once E. coli O157:H7 is suspected, diagnostic testing methods currently available require about a day to complete before a definitive diagnosis can be confirmed. Any means for shortening this dangerous delay would reduce the risk to infected patients and family members who might become infected through person to person transmission.
Of utmost importance for controlling outbreaks of E. coli O157:H7 is the rapid detection of the contaminated food source. A common method for detecting E. coli O157:H7 includes incubation on agar containing sorbitol, a substrate that supports growth of most fecal E. coli strains, but which the O157:H7 strain cannot metabolize. However, failure to ferment sorbitol does not provide a definitive diagnosis, as non-pathogenic strains of E. coli exist that also cannot ferment sorbitol.
A latex agglutination test for identification of the serogroup O157 is available from Oxoid (Unipath Limited, Basingstoke, Hampshire, England), which recommends that the test be applied to isolates that already have been determined to lack the ability to ferment sorbitol. This antibody-based test lacks specificity: Some strains of E. hermnaii share the antigen detected by the antiserum; hence this test must be confirmed by further fermentation testing before an isolate can be conclusively identified as E. coli O157:H7. Furthermore, noncytotoxic E. coli may possess the E. coli O157 antigen, leading to false positive reactions in such 0157 antigen detection systems.
Other diagnostic methods for E. coli O157:H7 have been suggested. Immunofluorescent examination of fresh stool with labeled O157-specific antiserum has recently been proposed as a rapid technique for the detection of patients whose stools contain E. coli O157:H7 (Park et al., Am. J. Clin. Path. 101:91-94, 1994). Antibody-coated magnetic beads (Dynal) are available that react with E. coli O157:H7 However, antigen detection tests should be accompanied by a confirmatory stool culture (Tarr, Clinical Infectious Diseases, 20:1-8, 1995).
A test kit for E. coli O157:H7 is available which reportedly can detect this organism in meat (Organon Teknika, Durham, N.C.). This is an antigen-based technology, in which bacterial antigens are used as targets to identify the organism. The test requires a day (i.e., overnight) to produce a presumptive positive test. An additional technique which has been proposed is based on a rapid dipstick immunoassay to detect enterohemorrhagic E. coli O157:H7 in retail ground beef (Kim et al., Applied Environmental Micobiology 58: 1764-1767, 1992).
For diagnostic purposes, conventional microbiological and antibody-based testing techniques may be insufficient for management of epidemic outbursts of HUS. By the time HUS appears, about two-thirds of patients no longer have E. coli O157:H7 in their stools. Moreover, the numbers of organisms present in contaminated food samples often are too low to be readily detected. DNA probes specific for this pathogen would provide an alternative method that is capable of detecting small numbers of organisms. Most efforts in the past to develop such probes have focused on the gene encoding the Shiga-like toxins (SLTs) believed to be responsible for many of strain O157's pathogenic effects. See: Levine et at., The Journal of infectious Diseases 156(1):175-182, 1987; Samadpour et al., Applied and Environmental Microbiology 56(5):1212-1215, 1990; Pollard et al., Journal of Clinical Microbiology 28(3):540-545, 1990; Pollard et al., The Journal of Infectious Diseases 162:1195-1198, 1990. However, the pathogenecity of many strains containing SLT genes is questionable (Tarr, Clinical Infectious Diseases, 20:1-8, 1995).
PCR assays to detect O157:H7 strains using the uidA gene have also been reported (Feng et at., Applied and Environmental Microbiology 57(1):320-323, 1991; Feng, Molecular and Cellular Probes 7:151-154, 1993.).
More recently, a multiplex PCR assay was reported which uses three sets of primers, two of which are directed to conserved regions within genes encoding for SLT-I and SLT-II, and the third set directed to the uidA gene (Cebula et at., Journal of Clinical Microbiology 33(1):248-250, 1995).